Objection 1: It seems that truth is not a part of justice. For it seems proper to justice to give another man his due. But, by telling the truth, one does not seem to give another man his due, as is the case in all the foregoing parts of justice. Therefore truth is not a part of justice.
Obj. 2: Further, truth pertains to the intellect: whereas justice is in the will, as stated above (Q. 58, A. 4). Therefore truth is not a part of justice.
Obj. 3: Further, according to Jerome truth is threefold, namely, "truth of life," "truth of justice," and "truth of doctrine." But none of these is a part of justice. For truth of life comprises all virtues, as stated above (A. 2, ad 3): truth of justice is the same as justice, so that it is not one of its parts; and truth of doctrine belongs rather to the intellectual virtues. Therefore truth is nowise a part of justice.
On the contrary, Tully (De Invent. Rhet. ii) reckons truth among the parts of justice.
I answer that, As stated above (Q. 80), a virtue is annexed to justice, as secondary to a principal virtue, through having something in common with justice, while falling short from the perfect virtue thereof. Now the virtue of truth has two things in common with justice. In the first place it is directed to another, since the manifestation, which we have stated to be an act of truth, is directed to another, inasmuch as one person manifests to another the things that concern himself. In the second place, justice sets up a certain equality between things, and this the virtue of truth does also, for it equals signs to the things which concern man himself. Nevertheless it falls short of the proper aspect of justice, as to the notion of debt: for this virtue does not regard legal debt, which justice considers, but rather the moral debt, in so far as, out of equity, one man owes another a manifestation of the truth. Therefore truth is a part of justice, being annexed thereto as a secondary virtue to its principal.
Reply Obj. 1: Since man is a social animal, one man naturally owes another whatever is necessary for the preservation of human society. Now it would be impossible for men to live together, unless they believed one another, as declaring the truth one to another. Hence the virtue of truth does, in a manner, regard something as being due.
Reply Obj. 2: Truth, as known, belongs to the intellect. But man, by his own will, whereby he uses both habits and members, utters external signs in order to manifest the truth, and in this way the manifestation of the truth is an act of the will.
Reply Obj. 3: The truth of which we are speaking now differs from the truth of life, as stated in the preceding A. 2, ad 3.
We speak of the truth of justice in two ways. In one way we refer to the fact that justice itself is a certain rectitude regulated according to the rule of the divine law; and in this way the truth of justice differs from the truth of life, because by the truth of life a man lives aright in himself, whereas by the truth of justice a man observes the rectitude of the law in those judgments which refer to another man: and in this sense the truth of justice has nothing to do with the truth of which we speak now, as neither has the truth of life. In another way the truth of justice may be understood as referring to the fact that, out of justice, a man manifests the truth, as for instance when a man confesses the truth, or gives true evidence in a court of justice. This truth is a particular act of justice, and does not pertain directly to this truth of which we are now speaking, because, to wit, in this manifestation of the truth a man's chief intention is to give another man his due. Hence the Philosopher says (Ethic. iv, 7) in describing this virtue: "We are not speaking of one who is truthful in his agreements, nor does this apply to matters in which justice or injustice is questioned."
The truth of doctrine consists in a certain manifestation of truths relating to science wherefore neither does this truth directly pertain to this virtue, but only that truth whereby a man, both in life and in speech, shows himself to be such as he is, and the things that concern him, not other, and neither greater nor less, than they are. Nevertheless since truths of science, as known by us, are something concerning us, and pertain to this virtue, in this sense the truth of doctrine may pertain to this virtue, as well as any other kind of truth whereby a man manifests, by word or deed, what he knows.